fix: stop mail loops from bounces and autoresponders

helpdesk/overrides/email_account.py

@@ -9,6 +9,41 @@
 from frappe.email.receive import InboundMail
 
 
+def auto_generated_reason(msg) -> str | None:
+    """Why this mail is machine-generated (bounce or autoresponder), else None.
+
+    One such mail can start three different loops: opening a ticket acks
+    raised_by straight back to the address that just bounced
+    (HD Ticket.after_insert), threading onto a portal ticket makes frappe CC the
+    parent doc's owner -- the same dead address -- on every inbound mail
+    (mail_cc), and an account with enable_auto_reply answers mailer-daemon.
+
+    X-Auto-Generated only catches helpdesk's own acks, which stamp it. Real
+    bounces announce themselves with RFC 3834 Auto-Submitted, the RFC 3464
+    report type, or the RFC 5321 null return-path.
+    """
+    if msg.get("X-Auto-Generated"):
+        return "X-Auto-Generated"
+
+    # RFC 3834: "no" is the only human-sent value, and it may carry parameters
+    auto_submitted = (msg.get("Auto-Submitted") or "no").split(";")[0].strip().lower()
+    if auto_submitted != "no":
+        return f"Auto-Submitted: {auto_submitted}"
+
+    # RFC 3464 delivery status notification -- survives a stripped Return-Path
+    if (
+        msg.get_content_type() == "multipart/report"
+        and msg.get_param("report-type") == "delivery-status"
+    ):
+        return "delivery status notification"
+
+    # bounces MUST carry a null envelope sender (RFC 5321 §6.1)
+    if (msg.get("Return-Path") or "").strip() == "<>":
+        return "null return-path"
+
+    return None
+
+
 class CustomInboundMail(InboundMail):
     """
     Extend InboundMail with robust thread stitching for forwarded emails.
@@ -68,15 +103,19 @@ def process_mail(messages, append_to=None):
                         message.decode("utf-8", errors="replace")
                     )
 
-                    # Important: If the email is auto-generated, we do not create a ticket
-                    if _msg.get("X-Auto-Generated"):
-                        continue
-
                     uid = (
                         messages["uid_list"][index]
                         if messages.get("uid_list")
                         else None
                     )
+
+                    # Important: auto-generated mail must never reach a ticket, it
+                    # starts a mail loop. The fetch already marked it seen, so park
+                    # it in Unhandled Email instead of dropping it without a trace.
+                    if reason := auto_generated_reason(_msg):
+                        self.handle_bad_emails(uid, message, reason)
+                        continue
+
                     seen_status = messages.get("seen_status", {}).get(uid)
                     if self.email_sync_option != "UNSEEN" or seen_status != "SEEN":
                         _inbound_mail = CustomInboundMail(

helpdesk/overrides/test_email_account.py

@@ -0,0 +1,98 @@
+import unittest
+from email import message_from_string
+
+from helpdesk.overrides.email_account import auto_generated_reason
+
+# Real Gmail DSN headers -- the shape that caused the 95-mail loop on ticket 74703
+GMAIL_BOUNCE = """\
+Return-Path: <>
+From: Mail Delivery Subsystem <mailer-daemon@googlemail.com>
+To: support@frappe.io
+Subject: Delivery Status Notification (Failure)
+Auto-Submitted: auto-replied
+Content-Type: multipart/report; report-type=delivery-status; boundary="b"
+
+--b
+Content-Type: text/plain
+
+The email account that you tried to reach does not exist.
+--b--
+"""
+
+OOO_REPLY = """\
+Return-Path: <someone@example.com>
+From: Someone <someone@example.com>
+To: support@frappe.io
+Subject: Out of office
+Auto-Submitted: auto-replied
+
+Back on Monday.
+"""
+
+# same DSN reaching us over POP3/Frappe Mail, where no MTA added a Return-Path
+BARE_DSN = """\
+From: Mail Delivery Subsystem <mailer-daemon@example.com>
+To: support@frappe.io
+Subject: Undelivered Mail Returned to Sender
+Content-Type: multipart/report; report-type=delivery-status; boundary="b"
+
+--b
+Content-Type: text/plain
+
+Recipient address rejected: User unknown.
+--b--
+"""
+
+REAL_CUSTOMER_REPLY = """\
+Return-Path: <omkar@batt.science>
+From: Omkar <omkar@batt.science>
+To: support@frappe.io
+Subject: Re: URGENT : Tickets raised outside working hours
+
+Any update on this?
+"""
+
+
+class TestAutoGeneratedReason(unittest.TestCase):
+    def test_gmail_bounce_is_dropped(self):
+        self.assertTrue(auto_generated_reason(message_from_string(GMAIL_BOUNCE)))
+
+    def test_out_of_office_is_dropped(self):
+        self.assertTrue(auto_generated_reason(message_from_string(OOO_REPLY)))
+
+    def test_dsn_without_return_path_is_dropped(self):
+        # only the RFC 3464 report type is left to go on
+        self.assertEqual(
+            auto_generated_reason(message_from_string(BARE_DSN)),
+            "delivery status notification",
+        )
+
+    def test_legacy_x_auto_generated_still_dropped(self):
+        msg = message_from_string(REAL_CUSTOMER_REPLY)
+        msg["X-Auto-Generated"] = "auto-replied"
+        self.assertTrue(auto_generated_reason(msg))
+
+    def test_real_reply_is_kept(self):
+        self.assertIsNone(
+            auto_generated_reason(message_from_string(REAL_CUSTOMER_REPLY))
+        )
+
+    def test_missing_headers_do_not_look_like_a_bounce(self):
+        # no Return-Path / Auto-Submitted at all -- must not be mistaken for a DSN
+        msg = message_from_string("From: a@b.com\nTo: support@frappe.io\n\nhi")
+        self.assertIsNone(auto_generated_reason(msg))
+
+    def test_auto_submitted_no_is_kept(self):
+        msg = message_from_string(REAL_CUSTOMER_REPLY)
+        msg["Auto-Submitted"] = "no"
+        self.assertIsNone(auto_generated_reason(msg))
+
+    def test_auto_submitted_no_with_parameters_is_kept(self):
+        # RFC 3834 allows parameters after the value -- still human-sent
+        msg = message_from_string(REAL_CUSTOMER_REPLY)
+        msg["Auto-Submitted"] = "no; owner=someone@example.com"
+        self.assertIsNone(auto_generated_reason(msg))
+
+
+if __name__ == "__main__":
+    unittest.main()